At work we have a guy that draws screen mock-ups for us to view during design meetings. He'll design a new screen for our software, and provide us with a visual representation of what that will look like. He uses Microsoft Visio to create the drawings, and does an excellent job with them.
Today, he introduced me to gliffy. It's simply amazing. If you need to draw out a room arrangement, create flow charts, or basically create a graphical representation of any type of design, this is the place to go. It's free (supported by ads with a pay version available), and it works great. Its extremely easy to use. I was up and running in less than 5 minutes.
The coolest feature is that it automatically saves versions every few minutes. So, if you realize you need to go back to what the screen looked like 2 hours ago, and hitting undo 125 times isn't an option, simply click on one of the automatically saved versions and your back to where you need to be.
I love it. I don't work with this type software often, but occasionally I need it, and this will be the place for me from now on. You should try it out. I'm sure you'll love it to.
Oh, and it's definitely web 2.0 style. You can save it online (of course), and share it publicly on the web or privately with your friends and/or coworkers. You can imbed it into web pages, blogs, etc. seamlessly. It will take quite a bit to top this offering.